The invention relates to an arrangement and a method for the channel-dependent attenuation of the levels of a plurality of optical data channels.
It is known to use fiber amplifiers, for example erbium fiber amplifiers in optical wide area networks for the purpose of amplifying the optical signals. Such fiber amplifiers have the property of amplifying signals of different wavelengths to different extents. The result of this is that, e.g. in WDM systems (wavelength division multiplex), in which information is transmitted in parallel on a plurality of data channels of different wavelength in a fiber, the individual data channels are amplified to different extents depending on the carrier wavelength. On account of this amplification to different extents, it is necessary to equalize the individual data channels again after amplification using an attenuator unit per channel (equalizing).
The publication xe2x80x9cTunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiersxe2x80x9d, Kyo Inoue, Toshimi Kominatot, Hiromu Toba, IEEE Photonics Technology Letters, Vol. 3, No. 8, August 1991, p. 718-720, discloses attenuating in a channel-dependent manner, using a Mach-Zehnder interferometer, the optical signals of individual optical data channels that have been amplified to non-uniform extents by an amplifier, in order to obtain a uniform signal level on the individual channels. In this case, the Mach-Zehnder interferometer is configured as a filter which a attenuates higher wavelengths to a greater extent and thereby compensates for elevated amplification of these wavelengths by the preceding optical amplifier.
This attenuator arrangement has the disadvantage that its range of use is restricted to the specific application mentioned.
The present invention is based on the object of providing an arrangement and a method for the channel-dependent attenuation of the levels of a plurality of optical data channels which can be used in a multiplicity of applications.
Accordingly, the invention provides for an input signal of the plurality of data channels to be split in a wavelength-dependent manner between at least two optical paths, for the signal component of one of the optical paths to be attenuated and for the paths subsequently to be recombined. The invention is based on the concept of attenuating only a part of the input signal, which differs from the other part of the input signal in terms of its frequency composition, while the other part remains unchanged. Since only one of the signal components which differ in terms of their frequency spectrum is attenuated, spectral-dependent attenuation of the input signal which is composed of a plurality of optical signals of different wavelength is effected.
This opens up a multiplicity of possible applications. In particular it is possible for only individual channels to be attenuated, while other channels remain essentially unchanged. It is also possible for specific frequency ranges to be attenuated in a manner that is amplified in a targeted way, while other frequency ranges are not attenuated or are only slightly attenuated.
It is pointed out that it lies within the scope of the invention to perform attenuation of the signals on one or the other optical path. In this case, it may be expedient to provide suitable attenuating means on both paths, one or the other attenuating means being activated depending on the application, so that the spectrally different signals of one or the other path are attenuated.
In a preferred refinement of the invention, the signal is attenuated virtually wavelength-independently in the attenuating means, i.e. the transmission has a flat spectrum. The effect achieved as a result of this is that the spectral components of the respective path are attenuated to the same degree. The overall frequency-dependent attenuation is produced only after combination of the path having the attenuated signal component with the other path, whose signal component was not attenuated.
In an advantageous design of the invention, the filter means are designed in such a way that an upper channel of an upper wavelength is conducted completely through one path, a lower channel of a lower wavelength is conducted completely through the other path and the channels of the intervening wavelengths are conducted to a greater extent through one or the other path depending on their proximity to the upper and lower wavelength. After the signal attenuated in this way has been combined with the unattenuated signal of the other path, the resulting transmission of the overall arrangement is one which falls approximately linearly with respect to the wavelength.
In an alternative refinement, the filter means are designed in such a way that a middle channel of a middle wavelength is conducted completely through one path and the channels of the further wavelengths are conducted increasingly through the other path with increasing distance from the middle channel. This leads to the overall arrangement having a transmission which falls predominantly only in the middle wavelength range.
A further refinement consists in the filter means being designed in such a way that every n-th channel, for example every second channel, in each case being conducted through a different path. This makes it possible for every n-th channel to be attenuated in a targeted manner or even to be entirely masked out, thereby enabling add-drop applications.
The filter means are preferably realized by a first Mach-Zehnder interferometer having a first and a second arm of different lengths. The two arms merge with the first and second path. In this case, the first Mach-Zehnder interferometer operates as a filter and splits the input signal between two paths as a function of the wavelength. In a development of the invention, it is possible here for one of these paths to be split again between two paths by using a further Mach-Zehnder interferometer operating as a filter.
Preferably, it is provided that a first phase shifter is additionally located on one arm of the Mach-Zehnder interferometer operating as a filter. The use of a phase shifter means that the filter property can also easily be controlled, so that technology fluctuations, for example, can be compensated.
The attenuating means are preferably realized by a second Mach-Zehnder interferometer, which splits the signals of one path into two arms (Y-splitter) and recombines them downstream of the Mach-Zehnder interferometer (Y-combiner). In this case, although the two arms have the same length, a second phase shifter is located on one arm, via which phase shifter a phase shift of the two arms and hence an attenuation of the signal of the corresponding path can be set.
The phase shifter, like the phase shifter of the first Mach-Zehnder interferometer as well, is preferably formed by a heating electrode via which the,corresponding arm of the waveguide can be locally heated. This leads to an increase in refractive index in the heated region and thus to a phase shift.
Preferably, it is provided that the two paths between which the input signal is split by the first Mach-Zehnder interferometer are recombined in phase after the signal of one path has been attenuated. The signal of the other path has traversed a parallel waveguide without interference. In-phase combination of the two signal components avoids additional attenuation during the addition of the signals and hence optimizes the transmission of the entire structural part.
In this case, provision is preferably made for arranging a third phase shifter in the path in which the signal is not attenuated, via which phase shifter the phase difference between the two paths can be set in a targeted way and only the transmission of the entire structural part can be optimized or else attenuated in a targeted way.
The data channels used preferably lie in a wavelength range between 1530 nm and 1570 nm. By way of example, 40 data channels are realized in this range, and their carrier wavelength in the frequency domain in each case has a spacing of 100 GHz.
The arrangement is preferably of integrated optical design. In particular, the light signals of the data channels are transmitted in an integrated optical waveguide. A preferred application consists in the arrangement directly downstream of an optical amplifier that is likewise of integrated optical design, in order to compensate for the non-uniform optical gain of said amplifier.